Steam generation with reheat temperature control



Jain. 9, 1968 v. z. CARACRISTI 3,3

STEAM GENERATION WITH REHEAT TEMPERATURE CONTROL Filed Sept; 8, 1965 5 Sheets-Sheet 1 I f. T 4

m 30 "TI 36 I L I 7 f F F m F/C-B- 1 INVENTOQ 1 z. CAEACE/ST/ BYif/f% AGENT Jan. 9, 1968 v.z.cARAcR|sT|" 3,362,384

STEAM GENERATION WITH REHEAT TEMPERATURE CONTROL Filed Sept. 8, 1966 3 Sheets-Sheet 2 F/G 2 nvvs/vroz V- Z- CAZACB/ST/ United States Patent Ofiice 3,362,384- Patented Jan. 9, 1968 3,362,384 STEAM GENERATION WITH REHEAT TEMPERATURE CONTROL Vrrginrus Z. Caracristi, West Hartford, Conn., assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Sept. 8, 1966, Ser. No. 578,034 Claims. (Cl. 122-479) This invention relates to steam generating systems having steam reheating and in particular to a method and apparatus for controlling the reheat steam temperature.

The importance of steam temperature control is well known both insofar as it improves the steam cycle efficiency in a power plant and for protection of the steam turbine or other steam consuming apparatus. Many systems have been used in the past to regulate or control the temperature of the reheated steam in a reheat steam generator. These chiefly involve a method of varying the locations of heat absorption throughout the unit or by heat exchange between the primary and reheat steam. Heat exchange between the reheat steam and primary interstage superheater steam has been used for temperature control of the reheat. The closeness of the temperature level between the superheat and reheat has made such control sensitive to difierences in temperature head and requires extensive heat exchange surface. Primary steam at the final steam temperature has been used as the exclusive steam reheating medium in some cycles.

In my invention the superheater outlet steam at the final and ultimate steam temperature is used to transfer heat to low temperature reheat steam. The portion used for the heat exchanger is not the portion which is delivered to the superheat turbine but is a separately extracted portion. This extracted portion-i then returned to the steam generator or feedwater system at a location upstream of the steam generator. The amount of steam extracted from thethrough-flow system for this purpose is regulated to control the reheat steam temperature. The reheat surfaces are further located to maximize the efiicacy of this reheat temperature control.

It is in object of my invention to provide an improved method and appartus for reheat steam temperature control in a steam generating system.

Other objects and advantages of the invention will become apparent as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a steam generator utilizing the heat exchanger reheat temperature control with the spent heating medium being returned to the steam generator recirculating path;

FIG. 2 is a steam generating system with the spent heating medium being returned from the reheat control reheat exchanger by cascading it through the feedwater heaters and introducing it into the de-aerator; and

FIG. 3 is a schematic diagram of a steam generator with the heat exchanger located interstage steam reheating sections.

In the illustrative embodiment in FIG. 1 feedwater'is introduced through feedwater pipe 2 to economizer 4. The water which is heated in this economizer is passed through pipe 6 to mixing vessel 8 from which it is circuto the waterwall outlet header 18. Fuel is fired through burners 20 with combustion occurring in furnace 16 and the combustion products formed passing outwardly through flue 22. Thus the water in passing through the furnace wall tubes 14 is heated to steam, and the steam is then conveyed through pipe 24 to the low temperature superheater 26. Steam at an intermediate temperature is then passed through pipe 28 to the high temperature superheater as. At this point the steam leaves its final and ultimate steam temperature of 1000 F. and is conveyed through steam outlet pipe 32 to a high pressure superheat turbine (not shown). Steam exhausting from the turbine is returned to the boiler for reheating through the cold reheat line 34 and passes serially through the low temperature reheater section 36 and the high temperature reheater section 38. The reheated steam at its final steam temperature is conveyed through hot reheat line 40 to a low pressure reheat turbine (not shown). A portion of the flow entering waterwall outlet header 18 is induced through recirculating line 4-2 by the action of the recirculating pump 10.

A portion of the steam flow leaving the high temperature superheater 30 at full temperature is Withdrawn through pipe 44 and passed through the high pressure side 46 of heat exchanger 48. This heat exchanger can ideally be formed by forming high pressure side 46 as a spiral coil, while the heated side of the heat exchanger through which the reheat flows is generally open and, therefore, has a low pressure drop. The heat exchanger shell is simply a portion of the cold reheat piping thereby precluding the need for another pressure vessel. Inside this pipe the heat exchanger tubing is finned on the outsire, such as with welded spiral fins.

The second portion leaves the heat exchanger through return line 50 passing through the return pump 52 and is pumped back into the mixing vessel 8. Regulating valve 54 is located in pipe 44 so as to regulate the quantity of the second portion passing through the heat exchanger as the heating medium. This valve is operated by controller 56 which is responsive to the temperature of the steam leaving the reheater. The temperature of the final reheat steam i sensed by temperature transmitter 58 which conveys a control signal indicative of the temperature through control line 60 to set point 62. At this point the signal is compared to a signal which is representative of the desired temperature, and an error signal is then conveyed through control line 64 to the controller 5 6, which operates control valve 44 to regulate the flow through the heat exchanger to obtain the desired cycle reheat steam temperature.

The low temperature reheater 36 is at a location upstream with respect to the gas flow of the high temperature reheater section 38. When heat is transferred to the reheat steam through the heat exchanger, the temperature of the steam entering the low temperature reheater increases. Since this reheater section is located in a relatively high gas temperature zone, there is normally a large temperature head between the steam and the gas. Therefore, the percentage decrease in temperature head is much less at this location than it would be in a lower gas temperature region. It follows that the decrease in reheater pickup from the gas is minimized by this high gas temperature location. Also since there is some decrease in the heat absorption from the gas as it passes over the low temperature reheater section 36, the gas temperature approaching the high temperature reheater section 38 is lated through recirculating pump 10 to the waterwall inslightly increased. Consequently, a portion of the loss in gas pickup occasioned by the introduction of heat through the heat exchanger 48 is recovered in the high temperature reheater 33. This location of surfaces maximizing the utilization of the heat and the gas minimizes the amount 3 of heat which must be added for control purposes from the extracted primary steam portion.

Of course, the gas temperature leaving the high temperature reheater also tends to increase at this time. Therefore, the =low temperature superheater 26 and the economizer 4 tend to pick up more heat. This heat is then available to supply the increased heat absorption required on the primary side due to the heating of the extracted portion of flow in addition to the first portion which is conveyed directly to the turbine.

The first portion of the steam flow, which is conveyed to the turbine, is not used for the heat exchange but rather is separate from the extracted portion. Therefore the temperature of the steam entering the turbine is the same as the steam leaving the final superheater surface. This avoids the decrease in efliciency occasioned with a through-flow heat exchanger system by low temperature steam to the turbine, or alternately, need for designing the superheater surface of the steam generator for excessively high temperatures. When additional heat must be absorbed by the superheater due to the superheat-to-reheat heat exchange requirement, additional flow occurs through the critical high temperature section. This higher flow improves the film conductance and thereby provides for safer metal temperature in the tubing of the high pressure section. Since there is higher flow in the section, the inlet steam temperature to that section is lower than it would otherwise be for the same gas temperatures and gas weights, so that throughout the high temperature section the tubing metal temperatures are again lowered.

The extremely high temperature of the steam which is used for the heat exchanger provides considerable temperature head between 1000 F. superheat steam and the temperature of the cold reheat steam which varies from 490 F. at 30 percent load to 600 F. at full load. Therefore, relatively little surface is required in the heat exchanger which makes for an inexpensive and low pressure drop design heat exchanger. Also because of this high temperature diflerence, the flow quantity is the primary determinate of the heat exchange quantity while variations in the temperature of either fluid have relatively little effect.

The return of the spent fluid from the high pressure side 46 of the heat exchanger 48 to the mixing vessel avoids all heat loss since any heat remaining in this liquid is obviously returned to the recirculating circuit of the steam generator. It can also be seen that since no fluid passes outside of the steam generator, there is no decrease in cycle efiiciency. The only loss in efliciency will be due to the slight increase in gas temperature passing up the stack. As previously indicated however, the decreased pickup in the reheater results in an increased gas temperature leaving the reheater. Most of this increase is absorbed by the low temperature superheater 26 and the economizer 4. The air heater (not shown) is located downstream of the economizer, and this further minimizes the increase in temperature of the stack gases.

The steam generator illustrated in FIG. 2 is identical to that illustrated in FIG. 1 except for the return line 66 conveying the spent heating fluid from the heat exchanger 48. In this figure the high pressure superheat turbine 33 and the low pressure reheat turbine 41 are also shown. These turbines are connected to drive electric generator 43. Steam leaving the low pressure reheat turbine 41 passes to condenser 68.

This condensate is pumped by condensate pump 70 through the low pressure feedwater heaters 72 to the de-aerator 74. From this point high pressure feedwater pump 76 passes the feedwater through the high pressure heaters 78 to the feedwater supply pipe 2. These feedwater heaters are of the conventional type wherein extraction steam from the turbine sections is utilized for heating the feedwater.

The return from the high pressure heating side of the heat absorber 48 is conveyed through line 66 to the heating side of the high pressure heaters 78. This fluid heats the feedwater passing through the heaters with this fluid cascading through the heaters and being introduced into the de-aerator 74. Obviously, this fluid could be further cascaded through the low pressure heaters and returned to the condenser, if desired. While again there is no heat loss from this fluid which is used in the heat exchanger 48, the cycle efliciency is slightly decreased since the amount of extraction steam from the turbine, which is used in the feedwater heaters, will be decreased. At the same time, however, the actual generator output will be increased since the additional extraction steam is passing through the entire turbine rather than passing to the feedwater heaters.

In the illustrative embodiment of FIG. 3 the primary flow circuit from the inlet to the economizer 4 to the outlet of the high temperature superheater 30 is the same as that described in detail with reference to FIG. 1. The reheat surface and the relation of the heat exchanger thereto is, however, located diiferently. The reheat steam entering through the cold reheat line 34 passes through a low temperature reheater which is located downstream of the low temperature superheater 26 and upstream of the economizer 4 with respect to gas flow. The reheat steam leaving this low temperature reheater through pipe 82 is passed through the heated side of the heat exchanger 48 and thence to the high temperature reheater 84. The reheat steam at the final outlet temperature is then conveyed to the turbine through the hot reheat line 40.

The heat exchanger 48 is located between reheater stages in this embodiment. It is possible to utilize this location while still maintaining a good temperature difference in the heat exchanger due to the extremely high temperature of the steam being used as the heating medium. The heat exchanger must, of course, be somewhat larger than that required for the other embodiments. The advantage of this arrangement lies in the ability of the low temperature reheater to continue to absorb heat from the gases at its maximum rate since the temperature head of the low temperature reheater is not detrimentally affected by transfer of heat in the heat exchanger 58. The use of this surface arrangement with my reheat temperature control system is of benefit when certain boiler design problems are met. For instance, where the furnace walls absorb so much heat that the temperature of the steam entering the low temperature superheater 26 is relatively high, it is difficult to find suflicient low temperature heat sinks to reduce the gas temperature. When such a design is encountered, it may be necessary to include a low temperature reheat section in this low gas temperature section. With such a location it is recommended that the heat exchanger be located between the reheater stages so as to destroy the efficiency of the low temperature reheater when there already tends to be a deficiency in reheat temperature.

While my invention has been described with reference to automatic controls for simplicity in description, it is apparent that the reheat steam temperature may be read, with valve 54 being controlled manually.

While I have illustrated and described a preferred embodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not Wish to be limited to the precise details set forth but desire to avail myself of such changes as fall Within the purview of my invention.

What I claim is:

1. A steam generating system comprising: a furnace; fuel burners for burning fuel within said furnace thereby forming a flow of combustion gases; at primary flow circuit for passing water in heat exchange relation with the burning fuel and generating primary steam; a primary flow super-heater for passing the primary steam in heat exchange relationship with the flow of combustion gases and superheating the primary steam to a final and ultimate steam temperature; means for delivering a first portion of said primary steam flow at the final steam temperture to a steam consuming apparatus; first reheating means for conveying a reheat steam flow in heat exchange relationship with the flow of combustion gases; a heat exchanger; means for conveying a second portion of the primary steam flow at the final steam temperature through said heat exchanger in heat exchange relationship with the reheat steam fiow at a location upstream of said first reheating means with respect to the reheat steam flow; regulating means for varying the fiow quantity of said second portion; means for determining the temperature of the reheat steam flow leaving said first reheating means; said regulating means being responsive to said temperature determining means :and controlling the temperature of the reheated steam to a desired value.

2. An apparatus as in claim 1 having also: means for conveying said second portion directly from said heat exchanger to said primary flow circuit at a location up stream of said primary flow superheater with respect to the primary flow.

3. An apparatus as in claim 1 wherein said first reheating means is comprised of a low temperature section and a high temperature section, with said loW temperature section being upstream of said high temperature section with respect to both gas flow and reheat steam flow.

4. An apparatus as in claim 1 having also: a plurality of feedwater heaters; a water through-flow path comprising said feedwater heaters being serially connected together and to said primary flow circuit for conveying a flow of water through said feedwater heaters for delivery to said primary flow circuit; means for conveying said second portion from said heat exchanger through said feedwater heaters in heat exchange relation with the water flowing therethrough, and subsequently returning said second portion to the water through-flow path at a location downstream of the feedwater heaters with respect to the water flow through said heaters.

5. An apparatus as in claim 1 having also: second reheating means at a location downstream of said first reheating means with respect to gas flow and upstream of said heat exchanger with respect to steam flow.

References Cited UNITED STATES PATENTS 2,685,280 8/1954 Blaskowski 122-479 2,968,156 1/1961 Pacault et a1 73 3,030,779 4/1962 Hryniszak et al. 60104 3,111,936 11/1963 Brunner 122-479 KENNETH W. SPRAGUE, Primary Examiner. 

1. A STEAM GENERATING SYSTEM COMPRISING: A FURNACE; FUEL BURNERS FOR BURNING FUEL WITHIN SAID FURNACE THEREBY FORMING A FLOW OF COMBUSTION GASES; A PRIMARY FLOW CIRCUIT FOR PASSING WATER IN HEAT EXCHANGE RELATION WITH THE BURNING FUEL AND GENERATING PRIMARY STEAM; A PRIMARY FLOW SUPER-HEATER FOR PASSING THE PRIMARY STEAM IN HEAT EXCHANGE RELATIONSHIP WITH THE FLOW OF COMBUSTION GASES AND SUPERHEATING THE PRIMARY STEAM TO A FINAL AND ULTIMATE STEAM TEMPERATURE; MEANS FOR DELIVERING A FIRST PORTION OF SAID PRIMARY STEAM FLOW AT THE FINAL STEAM TEMPERATURE TO A STEAM CONSUMING APPARATUS; FIRST REHEATING MEANS FOR CONVEYING A REHEAT STEAM FLOW IN HEAT EXCHANGE RELATIONSHIP WITH THE FLOW OF COMBUSTION GASES; A HEAT EXCHANGER; MEANS FOR CONVEYING A SECOND PORTION OF THE PRIMARY STEAM FLOW AT THE FINAL STEAM TEMPERATURE THROUGH SAID HEAT EXCHANGER IN HEAT EXCHANGE RELATIONSHIP WITH THE REHEAT STEAM FLOW AT A LOCATION UPSTREAM OF SAID FIRST REHEATING MEANS WITH RESPECT TO THE REHEAT STEAM FLOW; REGULATING MEANS FOR VARYING THE FLOW QUANTITY OF SAID SECOND PORTION; MEANS FOR DETERMINING THE TEMPERATURE OF THE REHEAT STEAM FLOW LEAVING SAID FIRST REHEATING MEANS; SAID REGULATING MEANS BEING RESPONSIVE TO SAID TEMPERATURE DETERMINING MEANS AND CONTROLLING THE TEMPERATURE OF THE REHEATED STEAM TO A DESIRED VALUE. 